1. Field of the Invention
The present invention relates to a charge/discharge control circuit for detecting a voltage and an abnormality of a secondary battery and to a battery device including the charge/discharge control circuit, and more particularly, to a charge/discharge control circuit capable of control by a single charge/discharge control MOSFET and to a battery device including the charge/discharge control circuit.
2. Description of the Related Art
FIG. 5 illustrates a circuit diagram of a battery device including a conventional charge/discharge control circuit. In the battery device including the conventional charge/discharge control circuit, an enhancement mode N-channel MOSFET 306 capable of bidirectional energization/interruption is connected in series to a negative terminal of a secondary battery 101. A charge circuit or a load is connected to terminals 120 and 121, and a charge/discharge current is supplied or discharged to or from the secondary battery 101 via the terminals 120 and 121. A control circuit 102 detects a voltage of the secondary battery 101 and a voltage of the enhancement mode N-channel MOSFET 306, and controls ON/OFF of switches 301, 304, and 305 based on the detected values. When a potential of a gate terminal of the enhancement mode N-channel MOSFET 306 is equal to or higher than a positive threshold voltage, the enhancement mode N-channel MOSFET 306 provides bidirectional energization between the drain terminal and the source terminal thereof. When the potential of the gate terminal is lower than the threshold voltage, the enhancement mode N-channel MOSFET 306 enters the OFF state between the drain terminal and the source terminal.
A charge-inhibited state is described. When a charger is connected between the terminals 120 and 121, a voltage Vds between the drain terminal and the source terminal of the enhancement mode N-channel MOSFET 306 has a positive value. The control circuit 102 detects that the voltage Vds is positive, and turns ON the switch 301 and OFF the switches 305 and 304. Accordingly, the gate terminal of the enhancement mode N-channel MOSFET 306 has a voltage higher than that of the source terminal thereof by the voltage of the secondary battery 101, with the result that the enhancement mode N-channel MOSFET 306 enters the energized state.
When the secondary battery 101 is charged and the battery voltage reaches a set upper limit value, the control circuit 102 turns OFF the switch 301 and ON the switches 305 and 304. Then, the gate terminal of the enhancement mode N-channel MOSFET 306 has the same potential as that of the source terminal thereof, with the result that the enhancement mode N-channel MOSFET 306 enters the OFF state. As a result, the charge current is interrupted to prevent overcharge of the secondary battery 101. Further, at this time, a diode 302 is reverse-biased to prevent the current from flowing through the switch 304 and the switch 305.
When the charge current is interrupted, no voltage drop by internal resistance occurs and the voltage of the secondary battery 101 reduces. In order to prevent the re-start of charge in response to the voltage reduction, after the charge is inhibited, the charge-inhibited state is maintained until the secondary battery 101 is discharged to some extent to have a voltage that is equal to or lower than a set value. Under the charge-inhibited state, if a load is connected between the terminals 120 and 121, the voltage Vds is switched from positive to negative. The control circuit 102 is thus configured to control the switches 301, 304, and 305 so that the secondary battery 101 may be discharged when the voltage Vds is negative and that the charge current may be interrupted when the voltage Vds is positive.
In the above description, the switches 304 and 305 are both turned ON at the time of the stop of charge. However, the charge can be stopped similarly even if the switch 304 is turned OFF. The first reason is that the switch 305 is ON regardless of ON/OFF of the switch 304, and hence the gate terminal of the enhancement mode N-channel MOSFET 306 has the same potential as that of the source terminal thereof and the enhancement mode N-channel MOSFET 306 thus enters the OFF state. The second reason is that the diode 302 also interrupts the current flowing through the switches 304 and 305.
Note that, the switches 304 and 305 are both OFF at the time of the charge described above and at the time of the discharge to be described later. Accordingly, if the switches 304 and 305 are both turned ON at the time of the stop of charge and the switches 304 and 305 are both turned ON also at the time of the stop of discharge as described later, the two switches are turned ON or OFF simultaneously all the time. It is therefore not necessary to control the switches 304 and 305 independently, which can simplify the configuration of the control circuit 102.
Next, a discharge-inhibited state is described. When a load is connected between the terminals 120 and 121, the voltage Vds between the drain terminal and the source terminal of the enhancement mode N-channel MOSFET 306 has a negative value. The control circuit 102 detects that the voltage Vds is negative, and turns ON the switch 301 and OFF the switches 304 and 305. Accordingly, the gate terminal of the enhancement mode N-channel MOSFET 306 has a voltage higher than that of the drain terminal thereof by the voltage of the secondary battery 101, with the result that the enhancement mode N-channel MOSFET 306 enters the energized state.
When the discharge of the secondary battery 101 progresses and the battery voltage reaches a set lower limit value, the control circuit 102 turns OFF the switch 301 and ON the switches 304 and 305. Then, the gate terminal of the enhancement mode N-channel MOSFET 306 has the same potential as that of the drain terminal thereof, with the result that the enhancement mode N-channel MOSFET 306 enters the OFF state. As a result, the discharge current is interrupted to prevent overdischarge of the secondary battery 101. Further, at this time, a diode 303 is reverse-biased to prevent the current from flowing through the switch 304 and the switch 305.
When the discharge current is interrupted, no voltage drop by internal resistance occurs and the voltage of the secondary battery 101 increases. In order to prevent the re-start of discharge in response to the voltage increase, after the discharge is inhibited, the discharge-inhibited state is maintained until the secondary battery 101 is charged to some extent to have a voltage that is equal to or higher than a set value. Under the discharge-inhibited state, if the charge circuit is connected between the terminals 120 and 121, the voltage Vds is switched from negative to positive. The control circuit 102 is thus configured to control the switches 301, 304, and 305 so that the secondary battery 101 may be charged when the voltage Vds is positive and that the discharge current may be interrupted when the voltage Vds is negative.
In the above description, the switches 304 and 305 are both turned ON at the time of the stop of discharge. However, the discharge can be stopped similarly even if the switch 305 is turned OFF. The first reason is that the switch 304 is ON regardless of ON/OFF of the switch 305, and hence the gate terminal of the enhancement mode N-channel MOSFET 306 has the same potential as that of the drain terminal thereof and the enhancement mode N-channel MOSFET 306 thus enters the OFF state. The second reason is that the diode 303 also interrupts the current flowing through the switches 305 and 304.
Note that, if the switches 304 and 305 are both turned ON at the time of the stop of discharge, as described above, the two switches are turned ON or OFF simultaneously all the time. It is therefore not necessary to control the switches 304 and 305 independently, which can simplify the configuration of the control circuit 102.
The enhancement mode N-channel MOSFET 306 has built-in diodes 321 and 322 formed therein. However, the diodes 321 and 322 are connected in series in opposite directions and hence are not electrically connected to each other, which has no influence on the protection operation described above.
The enhancement mode N-channel MOSFET 306 may be of either a lateral structure or a vertical structure. In the case of the lateral structure, it is easy to form the enhancement mode N-channel MOSFET 306 and the control circuit 102 as a single IC. Therefore, the reduction in size and cost can be achieved because the overcharge/overdischarge protection circuit, which has heretofore been formed by a single IC and two switches, can be formed by a single IC. On the other hand, in the case of the vertical structure, the reduction in loss can be achieved as compared to the lateral structure (see, for example, Japanese Patent Application Laid-open No. 2000-102182 (FIG. 9)).
The conventional technology, however, has a problem that the gate voltage of the enhancement mode N-channel MOSFET 306 can be reduced to no more than the source or drain voltage plus VF (about 0.6 V), and hence a leakage current is large when the enhancement mode N-channel MOSFET 306 is OFF. Further, the back gate of the enhancement mode N-channel MOSFET 306 becomes a floating state, leading to another problem of unstable operation of the battery device including the charge/discharge control circuit.